Decoder for multiple carrier pulse code modulation signals



April 17, 1951 R. l.. CARBREY 2,549,422

DECODER EoR MULTIPLE CARRIER PULSE coDE MoDuLATIoN SIGNALS Filed Jan 6, 1949 2 Sheets-Sheet l /56 l/ 4 26 'L /G 26 26 26 6R94 DELAY sf/R 0.9/ ,u sEc. DELAY -HFH DELA YV v A *y v r 96 CHAN/VEL PCM /NPUT 28\ 249` 28- 26` FROM FREQ. MUL T/PLEX .4. c6222 .4 gr; .A 275 .4 gif/1;

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DECODER FOR MULTIPLE CARRIER PULSE CODE MODULATION SIGNALS Filed Jan. G, 1949 2 Sheets-Sheet 2 U26 DELAY` DELAY 26 HG2 26 DELAY lav Ziv ar 28V! l zjavZav a GA TE GA TE GA TE 1 GA TE GA TE GA TE c.E.5 c.E. 4 c.E. s c.E. 2 d c.E.

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A 7' TOP/VE V l Patented Apr. 17,1951

DECODER FOR MULTIPLE oARmEaPULsE CODE MODULATION SIGNALS Robert L. oarbrey, summit, N. J., assignor to Beil Telephone VLaboratories, Incorporated, New

or values.

York, N. Y., a corporation of New York Application -lanuary 6, 1949, Serial No. v69,528

e 1 This invention relates to receiving equipment for decoding multichannel Vpulse code modulation.

In communication systems utilizing what is graph codes.

of two values.

an off pulse.

code.

tedby either time or frequency division multi- 9 ClaiInS. (Cl. 179-15) plex. Conveniently, and in many of the known codelpulse modulation systems the code group pulses are transmitted by time division multiplex and in some instances the code element pulses There is, however, a limit to the Iknown as pulse code modulation, a speech wave 5 for a plurality of different messages are transor other signal to ybe transmitted is sampled mitted overr the same system by interleaving periodically to ascertain its `instantaneous amthem in time.

plitude. The measured instantaneous amplitude extent to which this process may be utilized since is expressed by pulse codes analogous to teleit is a fundamental principle of time division 10 transmission that message signals must be sam- One code which conveniently may be employed pled at a repetition rate which is at least twice in pulse code modulation involves permutations the reqlleley f the highest Component Which of a fixed number of code elements each of it is desired to transmit. As accommodation for which may have any one of several conditions additional messages is added by time division it An advantageous code of this type l becomes necessary to shorten the code element is the so-called binary code in which each of the pulses with the ultimate result that impracfixed number of code elements may have either eebly Short 00de element pulses are required.

One way of representing these One means of circumventing this limitation invalues is to represent one by a pulse sometimes volves'the transmission of pulse code modulation referred to as an on pulse and the other by 2O signals by a COmbiIlelOn 0f time and frequency the absence of a pulse sometimes referred to as dViSiOIl multiplex, Thus, fOr eXemPle, if it iS Alternatively, one value may be Ydesired to transmit 384 different messages, the represented by a positive pulse and the other messages are interleaved by time division in bya negative pulse. The total number ofpermugroups of 96 messages each and the necessary tations obtainable with the binary code is profour groups of time division message channels are portional to 2n where n 'is the number bf code transmitted over four different freuuency'chan` elements employed. A f 'K nels. This may be accomplished by providing a Because the total number of different ampliseparate coding device for each of the frequency tudes which may be represented by such a code channels, each of these coding devices being of a xed number of elementsislimited, it is capable of operating at a suiiiciently high speed found desirable to divide the continuous range to handle 96 diiferent message signals. The reof amplitude values of which the transmitted sigceiver vfor such a transmission system could con- Anal is capable into a xed number of constituent ceivably comprise a separate decoder for each ranges which together encompass the total range. frequency channel with its group of 96 message Each of these smaller or constituent amplitude Signals andin many inStahCeS it may be desirable ranges may then be treated as if it were a single Y to employ two or more decoders for each 96 mes-l amplitude instead of `a range and is represented sage signal group. However, certain decoding by an individual one of the permutations of the circuits have been devisedvv which are capable of In the use of this method of pulse code accepting and decoding message signal code modulation the instantaneous amplitude ascer- 40 groups at very high rates of speed. tained by a sampling operation is represented by Accordingly, it is the object of the present inthe respective permutation indicative of the amvention to provide decoding equipment wherein plitude range," or step which' most nearly apa single high speed decoder is arranged to accept proximates the amplitude of the message sample. and decode the information transmitted over a If, for example, the sample amplitude is'nearest 45, plurality of different frequency channels each of to that amplitude represented by the ninth step which carries a pluralityy of different messages of the signal amplitude range the permutation transmitted by time division. code corresponding to range 9 is transmitted. In the decoding equipment in accordance with It will be observed', that each code element in the invention, apparatus is provided for acceptone of its values repreesnts the presence in saming the pulses of a code group simultaneously in pled amplitude of a particular Xed portion of separate input circuits and providing a reprethe total amplitude range, while in the other sentative output for each such code group. Correvalue it represents the absence of that same porspending code groups occurring in the several tion. The code element pulses may be transmitfrequency division transmission links are applied to the single decoding circuit by rstvadjusting J the times of occurrence of the code groups in each of the links in relation to those in the other links so that they are staggered in time by iractions of the period of a single code group. Preferably they are so staggered that corresponding code element pulses from the several links are equally spaced in a period equal to the code group period, The pulses of the code groups of each of the links are then applied to separate distributors which make the pulses available simultaneously in separate output circuits and the output circuits for each code element for each transmission link are connected to the corre spondng input circuits of the single decoding apparatus.

The above and other features of the invention will be described in detail in thefollowing specification taken in connection with the drawings in which:

Figs. 1 and 2 when placed together, with Fig. 2

-to the right of Fig. l, comprise a block schematic diagram o decoding apparatus in accordance with the invention.

In the decoding equipment shown by way of example in the drawings provision is made for accepting and decoding the pulse code modulan tion signals corresponding to 384 different messages, these code groups being transmitted over four frequency division links, each of which carries'the code groups for 96 different messages on a'time division multiplex basis. lt is assumed "that the '96 message channels for each frequency division link are transmitted consecutively and `that corresponding code groups in each link are transmitted simultaneously. It is further as sumedthat each code group comprises l0 code elements and that in these code groups the code element representing the highest denominational order of the code is transmitted first, followed consecutively by the code elements representing successively lower denominational orders. IThus, in each code 'group the iirst pulse transmitted represents the presence or absence of one-half the total possible amplitude range of which the `message wave is capable, while the last code element transmitted' represents the presence or absence oi one l02flth part oi' the total amplitude range.

The input signals from the four frequency division transmission links IG, i2, lil and IS are applied to pulse-shaping equipment i8 which is provided for each Vlink and comprises clipping, limiting or slicing circuits arranged to improve the pulse form andto remove any distortions as `to pulse'timingintroduced in the transmission link. The pulse outputs of the four shaping circuits l8 are 'applied to equipment arranged to stagger corresponding pulses of corresponding code groups to occur at equal intervals throughout an interval equal tothe period of one such code group. If a sampling rate of 8 kilocycles per second is assumed, the multiplex frame is 125 microseconds, each code group in each 96- channel link has allotted to it a period of 1.3 microseconds, and each of the ten code element pulses or these code groups occupies an interval of 0.13 microsecond. As shown, the corresponding pulses of corresponding code groups appearing at the outputs of the four pulse-shaping circuits l8 are staggered in time by a series of delay lines. If the pulses from link I!) are taken as a reference, those from links l2, lll and l are subjected to delays equal respectively to 0.33, 0.65 and 0.97 microsecond introduced by delay lines Thus, corresponding pulses of corresponding code groups from the four transmission links are equally spaced throughout a 1.3 microsecond code group period. In the general case Where code groups of period t are distributed ior transmission among m transmission links, delay devices are provided to adjust the times of the pulses from the m links so that with respect to the code element pulses i'rom one link, corresponding code element pulses from the remaining links follow at intervals equal respectively to (i112 m m The circuits associated with each of delay lines Eil, 22 and and that associated directly with the output of the shaping circuit I8 for link I0 are identical in every respect and only that associated with delay line 243 will be considered in detail. The delayed code element pulses appearing in the output of delay llnei are applied to a collecting'circuit comprising (1L-l) delay devices 25 connected in series and each capable of introducing a delay equal to the 0.13-miciosecond interval assigned to each code element of a code group. The delay line comprising the tandem connection of these delay circuits is terminated in its characteristic impedance Zo to prevent spurious reflections. Connections are made at the outputs of delay line and each of delay devices 6 and a gating or switching circuit 28 is connected in each of the output circuits so provided. Each of switching circuits 23 may com prise a vacuum tube having at least two control grids. The output signal from the associated delay circuit is applied to one of the grids and the tube is enabled or gated by an auxiliary gat ing pulse applied to the other grid to produce an output signal proportional to the input signal applied to the first-mentioned grid. It will be recognized that when the consecutive pulses of a code group are applied to the delay line comprising delay devices 26, each of the pulses will travel down the line, being delayed by an interval equal to the puise interval each time a delay netw rk 25 is traversed. Consequently, at the time at which the final pulse appears at the output of group delay linei, the ten pulses of the code group are available simultaneously at the input connections to gating circuits 28. If the gating circuits are simultaneously enabled at this time, output pulses corresponding to the ten code elements will appear simultaneously in separate circuits represented by leads 3l through 5B connected to the outputs of the ten gate circuits Z8.

The corresponding code groups from transmission links lll, I2 and Ul are applied to similar collecting and gating equipment for each link with the result that the code element pulses of corresponding code groups of these channels may also be made available simultaneously on leads 35 through 53 at certain intervals. If the code element pulses of a particular code group from transmission link il) are chosen as a reference and are caused to appear simultaneously on leads S through lill at a particular instant by a gating4 pulse .applied to the ten gate circuits associated with that link at that instant, the ten code elements of the corresponding code group from link l2 can be made available by enabling the gates associated with that link simultaneously 0.33 microsecond later, while the corresponding pulses for links lll and I6 may be made available by enabling the corresponding gate circuits successively at intervals of approximately 0.3 microsecond.

The gate circuits 28 may be enabled by ypulses from Ya pulse generator 32 which may, for example, comprise a conventional multivibrator as shown at page 512 of Radio Engineers Handboo byF. E. Terman having a repetition rate of 768 kilocycles per second and arranged to produce pulses of duration which is short with respect to that of the code element pulses. This pulse generator may be synchronized with the transmitter by known methods. These pulses, which occur at the code group rate for any link, may be applied directly without delay to the gate circuits associated with frequency link I and may be applied at the requisite intervals to the gate circuits associated with the remaining frequency channels through the Vuse of delay lines 34, 36 and 38 introducing delays which are respectively equal to those introduced in the code element pulses by delay lines 20, 22and 24.

The equipment thus `far described thus operates to accept simultaneously four -QB-channel J time division input signals and to distribute vthe code element pulses thereof among the ten leads 30 through 50 in such fashion that pulses corresponding to the 384 code groups appear code group by code group simultaneously and consecufollowed by the second code group of transmission link l0.

Leads`30 through 50 are connected to the ten Y 52," 54, 56, 58, 63, 62, 64,. 66, 68 and 10 equal in number to the vcode elements of the code groups and connected in series between a, source of positiveY potential here shown as a battery 'l2 and an output circuit which may comprise a cathode 'follower 14. The battery 12 has a potentialequal to the total possible amplitude represented by the code and the resistances are so proportioned that the total resistances between the source of poten- '.tial and the ends of successive resistors remote therefrom are related as increasing powers of two. Constant current circuits v1li which may lconveniently comprise pentode-typek vacuum tubes having unbypassed cathode resistors as disclosed, for example, in Patent 2,180,364 to E'. R. Norton, November 21, 1939, are connected to the ends of the resistors of the network remotefrom the source of positive potential and are arranged 'normally to draw equal currents from said Source through the corresponding portions of theA resistance network. Thus, initially, the constant current circuit for the nal code element corresponding to the smallest increment of amplitude `of Whichthecodeis capable draws one unit of current through a total resistance of R, while the constant current circuits corresponding to code elements of successively higher denominational Vorder draw currents to produce drops of 2R, 4R,

BR, ISR, 32R, GGR, |28R,y 256R and 512B, respectively. The total drop thus produced between the source of positive potential B0 and the output circuit 62 is thus equal to |023 R correspond-- ing to the amplitude range, (v0 to 1023 units in- ;clusive) of which the code is capable.

. 6 The constant current circuits 16 are turned olf by on pulses appearing on the corresponding leads 30 through'. Since, however, the gatingJ circuits are enabled to produce extremely short output pulses for Vthe purpose of eliminating distortions produced during transmission and by delay devices 26, it is convenient first to lengthen the'pulses from leads 30 through 50 to insurecertain operation of the constant` current circuits. The pulses are thus applied to pulse lengthening circuits 18 which may comprise the combination of a number of pulse length doublers of the type disclosed in copending application Serial No. 577,954, filed February 15, 1945, now Patent No. 2,457,559 to Gr. H. Huber, December 28, 1948, wherein. the pulses are applied to a, lshort-circuited delay line of such length that pulses are reilected back to the input after a delay equal to the original pulse length and added to the original pulses to obtain pulses of twice the original length.

When on pulses appear upon'one or more of leads 30 through 50, the corresponding constant current circuits are' turned off and cease to draw currents from the source of potential 12 through the corresponding portions of the resistance network. The total drop is thus reduced in proportion to the portion of the total possible amplitude represented by the on pulses of the code group appearing on leads 30 through 50 at any particular time and the remainder, being the battery potential less this drop and corresponding to the sample amplitude, appears in the output amplifier 14.

Since the code groups of the four 96-channel links appear one after the other on leads 30 through 50, output pulses appear consecutively in output cathode follower M corresponding to the instantaneous amplitudes of the 384 message signalsVv and may be applied through suitable distributing equipment to a like number of output circuits. It is thus apparent that a single high speed decoding circuit may be utilized to decode a very large number of message channels, or to decode a signal from a very wide `band source, such as television, which was transmitted as time division groups over a pluralityl of frequency multiplexed channels.

.Inthe above, it has been assumed that corresponding code groups of the four frequency division channels are transmittedY Simultaneously.

VIt is obvious, of course, that if these code groups are not transmitted simultaneously suitable ad- `justments may be made in the values of delay lines 20, `22 and 24 lto bring corresponding pulses of corresponding code groups into time coincidence for application to the several pulse collecting delay line networks.

What is claimed is:

1. Apparatus for decoding a plurality of code groups each of n equally spaced codeelement pulses of equal length, said code groups being distributed amonga plurality of separate transmission channels comprising, a decoder arranged to accept the n pulses of a code group simultaneously on separate input connections and to 'produce an output corresponding to said code group, means for spacing corresponding pulses of corresponding code groups from said separate channels to occur at equal intervals during a period equal to that occupied by a code group in said channels, means at the output of said spacing means of each'channel for obtaining the 'n pulses of the code groups therefrom simultaneously in n separate circuits, and means for applying the pulses from the n simultaneous outputs or" each of said last-mentioned means to the corresponding input connections oi said decoder.

2 Apparatus for decoding a plurality orafi element code groups of frame period t, said code groups being distributed for transmission among m separate transmission links comprising an n element decoder arranged to accept the code element p-ulses of a code group simultaneously in separate input circuits and to produce an output corresponding to each code group, means for adjusting the code group pulses from the m links so that with respect to the code groups in one link corresponding code groups in the remaining links follow after intervals respectively equal to approximately means at the output or the adjusting means for each link for` collecting the pulses of each code group in that link into time coincidence and distributing them to separate circuits correspondingto the n code elements, and means connecting said separate circuits for each link for each code element to the corresponding input circuit of said simultaneous decoder.

3. Apparatus for` decoding a plurality of n element code groups of frame period 1;, said code groups being distributed for transmission among m separate transmission channels, m being equal to or less than 11., comprising an n element decoder arranged to receive the code element pulses of a code group simultaneously in separate input circuits and to produce an output corresponding to each code group, delay lines in the output of each of said transmission channels except one arranged to introduce delays in the pulses from said channels with respect to those from said one transmission channel equal respectively to approximately @lili arate circuits associated with each transmission channel for each code element to the corresponding input circuit of said decoder.

4. Apparatus for decoding a plurality of code groups each of n pulses occurring at equal intervals within a frame period t, said code groups being distributed among a plurality of separate transmission channels comprising, a decoder arranged to accept the n pulses of a code group simultaneously on separate input connections and to produce an output corresponding to each code group, means for spacing the corresponding pulses of corresponding code groups from said separate channels at equal intervals during a period equal to that or a code group in said channels, a series of delay devices connected in series to the output of said spacing means for each of said transmission channels to introduce in the pulses from said spacing means successive delays equal to the pulse intervals of said code groups, electronic switches connected to the output of each or said delay devices and an electronic switch connected to the output of said spacing means and said electronic switches being operative respectively to connect pulses occurring at said outputs to the n code element inputs of said decoder, and means associated with the switching devices for each of said transmission channels foroperating said switching devices at the conclusion of a code group from such channel.

5. Apparatus for decoding a plurality of code groups each of n pulses occurring at equal intervals within a frame period t, said code groups being distributed among a plurality of separate transmission channels comprising, a decoder arranged to accept the 1L pulses of a code group simultaneously on separate input connections and to produce an output corresponding to each code group, means for spacing the corresponding pulses of corresponding code groups from said separate channels at equal intervals during said frame period t, a series of delay devices connected in series to the output of said spacing means for each of said transmission channels to introduce successive delays equal to the pulse intervals of said code groups in pulses from said spacing means, gate circuits connected between the output of said spacing means and of said delay devices respectively and the corresponding n input connections of said decoder for each transmission channel, means for enabling the gate circuits for each transmission channel at the conclusion of a code group therefrom for an interval which is short as compared to the length of the code element pulses, and means in the inputs of said decoders for lengthening pulses applied thereto from said gate circuits.

6. Apparatus for decoding a plurality of code groups each of n equally spaced code element pulses of equal length, said code groups being distributed for transmission among a plurality of separate channels, comprising a decoder arranged to accept the 7L pulses of a code group simultaneously in separate input circuits and to produce a decoded output for each code group, means for spacing the corresponding pulses of corresponding code groups from said separate channels to occur at equal intervals during a period equal to that of a code group in said channels, (rt-1) delay devices connected to the output of each cf said channels and arranged to introduce successive delays equal to the pulse intervals .of said code groups, electronic switches connected respectively between the output of said spacing means and between the outputs of each of said delay lines of each channel and the n input circuits oi said decoder, and means for operating said switches for each channel simultaneously to apply the n pulses of decoder groups from a particular channel to the n input circuits of said decoder.

7. Apparatus for decoding a plurality of code groups each of n equally spaced pulses of equal length, said code groups being distirbuted for transmission among a plurality of separate channels, comprising means for spacing corresponding code element pulses of corresponding code groups from said separate channels to occur at equal intervals during a period equal to that occupied by a code group of said channels, means at the output of said spacing means for said channels for distributing the u pulses of the code groups therefrom to occur simultaneously in 1L separate circuits, means interconnecting the corresponding ones of said n separate circuits for each channel to a decoding circuit having n separate inputs corresponding to the code elements of said code groups and comprising n resistors connected in series between a source of potential and an output circuit, said resistors being proportioned to provide total resistance between 9 said source of potential and the ends of successive resistors remote from said source of potential related as increasing powers of two, a constant current circuit connected to the ends of each of said resistors remote from said source of potential and corresponding to said n code m separate channels, m being equal to or less than n, comprising an n element decoder arranged to accept the pulses of a code group simultaneously in separate input circuits and to produce a decoded output for each code group, delay lines in the outputs in each of said channels except one for introducing delays in the code pulses therefrom with respect to the pulses in said one channel equal respectively to approximately m m m a series of delay lines connected in the outputs of each of said rst-mentioned delay lines land arranged to introduce successive delays equal to the pulse intervals of said code group, a similar series of delay lines connected in the output of said one channel, gate circuits connected to the output of said one channel, the outputs of said first-mentioned delay lines and the outputs of each of said last-mentioned delay lines to complete circuits therefrom to the n respective input circuits of said decoder, a source of enabling pulses for said gate circuits occurring at the code group repetition rate for said one channel, means for applying said pulses to the gates associated with said one channel simultaneously, and means for delaying said pulses for applica- 10 tion to the gates of the remaining ones of said channels by the same amounts as the code pulses from said channels are delayed with respect to those from said one channel.

9. Apparatus for decoding a plurality of code groups each of 1L equally spaced code element s pulses of equal length, said code groups being distributed among a plurality of separate transmission channels comprising a decoder arranged to accept the n pulses of a code group simultaneously on separate input connections and to produce an output corresponding to said code group, means introducing delays to space corresponding pulses of corresponding code groups from said separate channels to occur at equal intervals during a period equal to that occupied by a code group in said channels, means at the output of said spacing means for each channel for obtaining the n pulses of the code groups therefrom simultaneously in 11, separate circuits, a gate circuit for each of the n separate circuits for each channel and means for enabling all gate circuits for each channel simultaneously to apply the simultaneous pulses for the code groups from each channel in turn to said decoder, said enabling means comprising a pulse generator operating at the code group repetition ratek for one of said channels and delay units introducing delays respectively equal to those produced by said'delay means connected between the output of said pulse generator and the gate circuits for the corresponding channels.

ROBERT L. CARBREY.

REFERENCES CITED The following references are'of record in the file of this patent:

UNITED STATES PATENTS Schelleng Nov. 9, 1948 

